Evohome behaviour with underfloor heating

[rh63]

A month ago I installed an evohome system in our house. The system is made up of 7 x HR92, the new Evotouch WIFI (latest firmware), HCE80 underfloor heating controller with external antenna and 5 zones and 2 x DTS92's to measure the underfloor zones. Our house has underfloor heating only on the ground floor (95m2), the 1st and 2nd floor are heated with radiators.
My boiler is a condensing combi boiler with opentherm (Nefit TrendlineII HR30C with opentherm adapter) and I'm using the Evohome 8810 opentherm boiler relay. Setting up the system was easy and so far it's running ok. Overshoot's are getting rare and it looks like the learning phase has worked.

What is still puzzling me though is the way Evohome handles the underfloor heating. Every now and then evohome starts the underfloor heating which causes the underfloor pump to start and the actuators to slowly (+- 2 minutes) open. I expected that the evohome would wait firing up the boiler until the actuators are open but it doesn't, it starts the boiler at the same time it starts the pump and the opening of the actuators. A lot of times the evohome stops the boiler even before the actuators are fully open, a waste of energy if you ask me, the boiler is a fast one and can deliver hot water for the heating within a minute.
The evohome is a smart system so why doesn't it take into account that the actuators need a (configurable?) amount of time before they are open? It should wait that period before firing up the boiler and pump. Or am I worrying to much and is this the best way to handle underfloor heating?

Thanks for the help,

richard

[DBMandrake]

Do you use a boiler control relay in your configuration ? If so have you tried increasing the minimum on time from 1 minute to 2 minutes ?

I notice a similar behaviour with HR92's - when you first turn the set point way up on an HR92 it will open itself first (which can take nearly 30 seconds from fully closed) and then send a boiler demand that causes the boiler relay to come on. However when closing down the HR92 does not do things in reverse like you might expect - it will slowly close the valve and only then when closed will it remove the boiler demand allowing the boiler relay to turn off.

During this time the boiler is potentially pumping through only the automatic bypass valve - hence the mandatory requirement for an automatic bypass valve on this system if you don't have a dedicated bypass radiator.

I expected that the evohome would wait firing up the boiler until the actuators are open but it doesn't, it starts the boiler at the same time it starts the pump and the opening of the actuators.

You could argue that there is some sense in doing that - as assuming you have an automatic bypass valve (which you should) that allows the boiler to start warming up in parallel with the opening of the valves.

As you say, it takes your boiler a full minute to warm up - and that may be relatively quick by boiler standards. If it waited a couple of minutes for your valve to finish opening and then fired the boiler and then you had to wait another minute or two for the boiler to produce any usable heat, that is a large chunk of a 10 minute TPI time period used up producing no heat, in every 10 minute cycle.

Doing the two actions in parallel will greatly accelerate the availability of heat in each TPI cycle at the expense of wasting a relatively tiny amount of it via the automatic bypass loop piping. (which won't loose that much heat in 1 or 2 minutes - my boiler flow temperature only drops about 10 degrees in 12 minutes with the burner off if it is pumping entirely via the ABV and not through any radiators)

A lot of times the evohome stops the boiler even before the actuators are fully open, a waste of energy if you ask me, the boiler is a fast one and can deliver hot water for the heating within a minute.

This is where you would want to increase the minimum on time, assuming you can in your configuration.

[The EVOHOME Shop]

Do you use a boiler control relay in your configuration ?

Richard is using the R8810 OpenTherm Bridge not a BDR91 Wireless Relay Box assigned as Boiler Control Relay, so temperature control of his boiler will be much more accurate and intuitive.

Every now and then evohome starts the underfloor heating which causes the underfloor pump to start and the actuators to slowly (+- 2 minutes) open. I expected that the evohome would wait firing up the boiler until the actuators are open but it doesn't, it starts the boiler at the same time it starts the pump and the opening of the actuators. A lot of times the evohome stops the boiler even before the actuators are fully open, a waste of energy if you ask me, the boiler is a fast one and can deliver hot water for the heating within a minute.
The evohome is a smart system so why doesn't it take into account that the actuators need a (configurable?) amount of time before they are open? It should wait that period before firing up the boiler and pump. Or am I worrying to much and is this the best way to handle underfloor heating?

This is pretty normal behaviour IMHO. Thermal actuators although not fully open will still pass a small amount of heating water into the circuit. evohome is expecting heat loss and when reaching set point will operate like this to maintain set point. The R8810 OpenTherm Bridge should be maintaining the flow temperature of the boiler quite accurately for the demand of the system, so overshoots in temperature should be minimal.

[rh63]

Thanks for the replies. There is an automatic bypass in the system so the late opening would not be a problem for the system I suppose. And I'm also not using the BRD91 relay but the opentherm bridge 8810 and it's maintaining the temperature quite nicely. I don't know if changing the minimum on time will help, I've tried it but as far as I understand from this forum the evohome doens't use this value when it's on an opentherm bridge? I couldn't see any different behaviour when I raised the minimum on time.

I don't think the actuators are fast enough because sometimes the boiler stops heating because it reaches it's maximum temperature and soon after that the temperature drops sharply when the actuators are opened and the floor comes "on-line". It would be nice to have a "opening delay" setting in the setup when a HCE80 is connected, maybe it's only usable with fast boilers but it would be nice to have.